The plateau surface of the central Burren is pitted with numerous enclosed depressions with internal drainage, of which that centred on Carran is the largest. The water sinking at the southern end of the depression had previously been traced to springs in the valley of the River Fergus to the south. The discovery in 2009, of a major, vertical cave (Poll Gonzo) near the northwestern extremity of the enclosed basin, stimulated new research in the area which is described in this paper. Positive water tracings from Poll Gonzo to submarine springs in Galway Bay and from the Castletown sink to the River Fergus valley springs show that the Carran depression lies on the divide for north and south draining groundwaters. The Carran enclosed depression is a complex karstic landform composed of seven sub-basins. Based on its morphology and hydrology it is suggested that the enclosed depression can be regarded as a small polje, an uncommon karst landform in temperate climates.